The present invention relates to leak detection in an automotive evaporative emission system, and more particularly to a detection method that takes into account the vapor generation characteristics of the system.
In automotive evaporative emission systems, fuel vapor generated in the vehicle fuel tank is captured in a charcoal-filled canister and subsequently supplied to the engine air intake through a solenoid purge valve. Since the effectiveness of the system can be significantly impaired by faulty operation of a component or by a leak in one or more of the hoses or components, the engine controller is generally programmed to carry out a number of diagnostic algorithms for detecting such failures. If faulty operation is detected, the result is stored and a xe2x80x9ccheck enginexe2x80x9d lamp is activated to alert the driver so that corrective action can be taken.
Experience has shown that evaporative system leaks can be particularly difficult to reliably detect and diagnose due to variability of fuel characteristics, driving schedules, and environmental conditions. While leaks can theoretically be detected by closing off the air vent, drawing the system below atmospheric pressure with engine vacuum, and then monitoring the change in system pressure, the results are subject to misinterpretation due to unmeasured effects such as vapor generation in the fuel tank. Accordingly, what is needed is a method of reliably detecting evaporative emission system leaks.
The present invention is directed to an improved method of detecting evaporative emission system leaks, wherein first and second changes in closed-system pressure due to vapor generation are measured respectively prior to and after the leak testing, and wherein the larger of the first and second pressure changes is used to adjust the pressure measurements taken during leak testing, or to invalidate the diagnostic if the vapor generation exceeds a threshold. The first vapor generation test occurs at the beginning of the driving cycle when there has been no significant disturbance of the vapor equilibrium in the fuel tank, and thereby provides an indication of vapor generation due to volatility of the fuel. The second vapor generation test occurs well into the driving cycle, and provides an indication of vapor generation due to fuel heating and sloshing.